Bruce, > If you look at the Allan deviation plot on the PRS10 page: > http://www.thinksrs.com/products/PRS10.htm > > This indicates that the likely disciplining loop time > constant will be several thousand seconds.
It should be noted that this plot shows a VERY PESSIMISTIC (almost two decades) behaviour of the GPS 1 pps, perhaps a remainder from SA and/or a not so well timing receiver. A M12+ would have its sawtooth corrected 1 pps at 2-4E-12 @ 1000 s, giving a crosspoint to the AD of the rubidium alone anywhere between 1000s and 10000 s. Best regards Ulrich > -----Ursprungliche Nachricht----- > Von: [email protected] > [mailto:[email protected]] Im Auftrag von Bruce Griffiths > Gesendet: Dienstag, 24. Februar 2009 21:38 > An: Markus Kern; Discussion of precise time and frequency measurement > Betreff: Re: [time-nuts] Different Thunderbolt versions > > > > Markus Kern wrote: > > On 22.02.2009, 21:12 Bruce Griffiths <[email protected]> > > wrote: > > > > > >> Markus > >> > > > > > >> Even with sawtooth correction the performance of the M12+T > was found > >> inadequate for the LOFAR > <http://www.lofar.org/p/systems.htm> array. > >> They use SRS FS725 rubidium sources disciplined by M12+T > GPS timing > >> receivers. > >> > > > > I didn't mean using the M12 by itself, obviously a clock > stable enough > > over the time the M12 pps must be integrated has to be used. > > > > If we are using the ADEV limits you proposed then at 50 MHz (= 3ns > > acceptable error) the timing requirement is an ADEV of > 3*1E-(8+x) at > > tau = x seconds. From the measurements at > > http://www.leapsecond.com/pages/gpsdo/ it seems the > Thunderbolt gets > > pretty close to that. > > > > The LOFAR clock system is described at [1]. In section 3.1.3.3 they > > say: > > > > "Some Crystal Oscillators have the advantage that they have > a better > > Allan variance for periods of up to 10s and therefore it can be > > claimed that they have a better performance than the SRS-FS725 > > Rb-reference standard. The performance for time periods > above 10s, the > > SRS-FS725 performs better. Therefore choosing an OCXO would > require a > > maximum calibration interval of 10s and it would require a > > significantly better GPS (or GALILEO) receiver because de > Rb-reference > > is used to average the PPS signal from the GPS receiver > thereby making > > it possible to identify the time difference between stations at > > receive frequencies above 10MHz." > > > > I think this means that they are using pps integration > times above 10 > > seconds. I couldn't find any reference to the actual value though. > > > > LOFAR is also working at frequencies up to 240MHz so the timing > > requirements are definitely higher. They say that a station time > > offset of 200ps does not affect performance as long as it remains > > stable over time. > > > > > >> They also state that the ionosphere contribution to ADEV is about > >> 8E-12 @10s. > >> > > > > Yes, from which they infer that "the reference clock shall have an > > Allan variance of 1e-11 or less over 10s." I am not sure if > this has > > to do with the propagation of the GPS signal or if they > mean that they > > need a clock stable enough to later compensate for the different > > delays of the observed signal through the ionosphere. > > > > Markus > > > > [1] > > > http://www.lofar.org/operations/lib/exe/fetch.php?id=public%3Adocument > > > s%3Alofar_documents&cache=cache&media=public:documents:19_detailed_des > > cription_of_clock_sync.pdf > > > > > > > > Markus > > The ionosphere contribution to the Allan deviation at GPS > frequencies is much smaller (by a factor of 10-100 or so) > than that, as is evident from carrier phase measurements. At > 50MHz the ionospheric phase shift, dispersion and instability > will be much greater than at GPS frequencies. They are merely > ensuring that the LO contribution to Allan deviation is much > smaller than that of the ionosphere. > > If you look at the Allan deviation plot on the PRS10 page: > http://www.thinksrs.com/products/PRS10.htm > > This indicates that the likely disciplining loop time > constant will be several thousand seconds. > > Close isn't good enough: the phase differences between pairs > of stations is significant, the Allan deviation needs to be > at least 30% lower per station. If the errors at station > pairs have significant correlation the requirement can be > relaxed somewhat. > > Bruce > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
